The present invention relates, most generally, to semiconductor opto-electronic devices and methods for forming the same. More particularly, the present invention relates to a method for enhancing the carrier concentration within a p-doped contact layer which contains arsenic.
Contact layers are formed over semiconductor opto-electronic devices to enable electrical contact to be made to the opto-electronic device through a medium which provides a suitably low contact resistance. High quality ohmic contact is needed between an external electrical wire and the opto-electronic device to allow for high device speed and to minimize the generation of heat in the opto-electronic device. To ensure ohmic contact and a suitably low contact resistance, a first requirement is a clean contact area between the electrical wire joined to the contact layer, and the surface of the contact layer to which it is joined. A second important requirement is for the contact layer to be a material having a suitably low band gap. For this reason, ternary materials such as InGaAs are commonly used. A third requirement for producing a suitably low resistance is a low sheet resistivity of the contact layer material. This is achieved by doping the contact layer with an impurity such as a P-type dopant. The P-type dopant species such as Zn, introduced into the contact layer, includes holes, or free carriers, which provide for the free flow of electrons, i.e. current within the contact layer material. Therefore, a commonly used contact layer material is p-InGaAs which includes Zn as the P-type dopant. Generally speaking, as a higher concentration of P-type dopant, and therefore a higher concentration of free carriers, is included within a contact layer, the sheet resistance and therefore the contact resistance is reduced. It is therefore desirable to incorporate as many free carriers as possible into the contact layer in order to minimize contact resistance.
One shortcoming associated with the incorporation of Zn or any other P-type dopant into a contact layer is that there is an associated saturation level or a physical imitation to the atomic concentration level of the P-type dopant which may be introduced into the contact layer.
A further shortcoming of the present technology and associated with the epitaxial formation of InGaAs as a contact layer, is that arsine, AsH3 is used as a process gas during the formation of the InGaAs contact layer. After the epitaxial process for forming the InGaAs film is completed, an amount of arsine gas is typically retained within the environment within which the contact layer is contained, in order to xe2x80x9cpreservexe2x80x9d the surface and prevent the unselective loss of arsenic from the layer. The drawback associated with the use of arsine gas in the environment after the film has been formed is that energized atomic hydrogen from the arsine gas passivates the P-type contact layer by complexing with the P-type dopants such as zinc (Zn) to form an electrically neutral Znxe2x80x94H pair.
The passivation of zinc carriers by atomic hydrogen is discussed in Cole, et al. Effect of Cooling Ambient on Electrical Activation of Dopants in MOVPE of InP, Electronics Letters 24, 930 (1988) and Antell, et al. Passivation of Zinc Acceptors in InP by Atomic Hydrogen coming from Arsine during Metalorganic Vapor Phase Epitaxy, Appl. Phys. Letters 53(9), 758, August 1988. The Cole article deals with the passivation of zinc carries on InP by atomic hydrogen.
By forming the electrically neutral pair thereby passivation the P-type dopant impurity, the holes or free-carriers associated with the P-type dopant are lost. In this manner, atomic hydrogen from the arsine gas passivates the surface by reducing the free carrier concentration within a film which includes P-type dopant impurities. This increases sheet resistivity and contact resistance. Thus, techniques such as post growth, post-cool down annealing have been developed to minimize such hydrogen passivation. Post growth, post-cool down annealing is discussed by Ishibashi, et al. in Annealing Effects on Hydrogen Passivaton of Zn Acceptors in AlGalnP with p-GaAs Cap Layer Grown by Metalorganic Vapor Phase Epitaxy, Journal of Crystal Growth, 145 (1994) 414-419. The Ishibashi, et al. article is directed to passivating underlying layers, not the exposed, upper contact layer.
Furthermore, as device geometries and film thicknesses continue to shrink, the thickness of conventional contact layers is correspondingly reduced. Since P-type dopant impurities such as Zn are typically introduced in-situ during the process of film formation, the concentration of the P-type dopant is relatively homogeneous throughout the formed film. It can therefore be seen that as the film thickness and therefore film volume decreases, the actual number of atomic P-type dopant impurities present in the layer is also diminished. Similarly, there are physically less holes or free carriers present as the thickness of the contact layer is decreased. The hydrogen passivation process described above occurs as atomic hydrogen enters the film through a top exposed surface, and the extent of hydrogen passivation is determined by the film growth process conditions and the cool-down conditions. As such, with contact layers being formed to increasingly smaller thicknesses, a greater percentage of the free carriers associated with P-type dopants, may be lost through passivation by hydrogen ions. Stated differently, the contact layers are more sensitive to hydrogen passivation as the film thickness of the contact layer decreases.
An object of the present invention is not only to prevent the above-described loss of free carriers from a p-doped contact layer by hydrogen passivation but, more importantly, to enhance the free carrier concentration above the atomic dopant concentration.
To address these and other needs, and in view of its purposes, the present invention provides a method for forming a p-doped contact layer over an opto-electronic device using metalorganic vapor phase epitaxy (MOVPE) methods. Arsine, AsH3, is included as a source gas in the epitaxial formation of the p-doped contact layer which is formed to include arsenic as a component. After the MOVPE process is completed and the p-doped contact layer is formed, the structure is cooled within an environment of reduced arsine concentration to preserve the surface of the contact layer until a threshold temperature within the range of 560xc2x0 C. to 580xc2x0 C. is attained, then the arsine is completely withdrawn from the environment to enhance the free carrier concentration of the P-type dopants above the atomic concentration of the P-type dopants. Contact resistance is thereby reduced.